Lewis Dot Structures

Lewis Dot Structures Gateway to Understanding Molecular Structure

Molecular Structure & Bonding A molecular structure, unlike a simple molecular formula, indicates the exact 3-D nature of the molecule. It indicates which atoms are bonded to which atoms, and the 3-D orientation of those atoms relative to each other.

Molecular Formula vs. Molecular Structure Molecular formula – H2O Molecular structure: .. .. O H H

Molecular Structure Two issues: • What is stuck to what? • How are they oriented?

What is stuck to what? The first thing you need to do in drawing a molecular structure is to figure out which atom sticks to which other atoms to generate a skeletal model of the molecule. The skeletal model is called a Lewis Dot Structure.

Lewis Dot Structures The first step towards establishing the full 3-D geometry of a molecule is determining what is stuck to what and how each atom is connected. Lewis Dot Structures provide this information.

Two Rules • Total # of valence electrons – the total number of valence electrons must be accounted for, no extras, none missing. • Octet Rule – every atom should have an octet (8) electrons associated with it. Hydrogen should only have 2 (a duet).

Total Number of Valence Electrons The total number of available valence electrons is just the sum of the number of valence electrons that each atom possesses (ignoring d-orbital electrons) So, for H2O, the total number of valence electrons = 2 x 1 (each H is 1s1) + 6 (O is 2s22p4) = 8 CO2 has a total number of valence electrons = 4 (C is 2s22p2) + 2 * 6 (O is 2s22p4) = 16

Determining the number of valence electrons: Full d-orbitals do not count as valence electrons. They belong to the inner shell. For example: As is [Ar]4s23d104p3 This is FIVE (5) valence electrons. The 3d is part of the inner shell (n=3) which is full.

How many valence electrons does Ge have? A. 12 B. 14 C. 3 D. 4 E. 5

! 1s2 2p6 3p6 4p6 5p6 6p6 7p6 2p5 3p5 4p5 5p5 6p5 7p5 2p1 3p1 4p1 5p1 6p1 7p1 2p2 3p2 4p2 5p2 6p2 7p2 2p3 3p3 4p3 5p3 6p3 7p3 2p4 3p4 4p4 5p4 6p4 7p4 1s1 2s1 3s1 4s1 5s1 6s1 7s1 2s2 3s2 4s2 5s2 6s2 7s2 3d1 4d1 5d1 6d1 3d5 4d5 5d5 6d5 3d7 4d7 5d7 6d7 3d8 4d8 5d8 6d8 3d2 4d2 5d2 6d2 3d4 4d4 5d4 6d4 3d9 4d9 5d9 6d9 3d10 4d10 5d10 6d10 3d3 4d3 5d3 6d3 3d6 4d6 5d6 6d6

Take a look at Ge electron structure [Ar]4s23d104p2 Full d-orbitals don’t count. So there are 4 valence electrons.

How many valence electrons does Ti have? A. 1 B. 2 C. 3 D. 4 E. 5

How many valence electrons does Te have? • 15 • 16 • 3 • 5 • 6

Central Atom In a molecule, there are only 2 types of atoms: • “central” – bonded to more than one other atom. • “terminal” – bonded to only one other atom. You can have more than one central atom in a molecule.

Bonds Bonds are pairs of shared electrons. Each bond has 2 electrons in it. You can have multiple bonds between the same 2 atoms. For example: C-O C=O C O Each of the lines represents 1 bond with 2 electrons in it.

Lewis Dot Structure Each electron is represented by a dot in the structure . :Cl: ¨ That symbol with the dots indicate a chlorine atom with 7 valence electrons.

Drawing Lewis Dot Structures • Determine the total number of valence electrons. • Determine which atom is the “central” atom. • Stick everything to the central atom using a single bond.

Dot structure for H2O 1. Total number of valence electrons: 6 + (2 x 1) =8 2. Central Atom – typically, the central atom will be leftmost and/or bottommost in the periodic table. It is the atom that wants more than one thing stuck to it. H is NEVER the central atom. 3. Stick all terminal atoms to the central atom using a single bond.

Dot structure for H2O H – O – H

Drawing Lewis Dot Structures • Determine the total number of valence electrons. • Determine which atom is the “central” atom. • Stick everything to the central atom using a single bond. • Fill the octet of every atom by adding dots. • Verify the total number of valence electrons in the structure.

Dot structure for H2O .. H – O – H ¨ That is a total of 8 valence electrons used: each bond is 2, and there are 2 non-bonding pairs.

Drawing Lewis Dot Structures • Determine the total number of valence electrons. • Determine which atom is the “central” atom. • Stick everything to the central atom using a single bond. • Fill the octet of every atom by adding dots. • Verify the total number of valence electrons in the structure. • Add or subtract electrons to the structure by making/breaking bonds to get the correct # of valence electrons. • Check the “formal charge” of each atom.

Formal Charge of an atom “Formal charge” isn’t a real charge. It’s a pseudo-charge on a single atom. Formal charge = number of valence electrons – number of bonds – number of non-bonding electrons. Formal charge (FC) is ideally 0, acceptably +/-1, on occasion +/- 2. The more 0s in a structure, the better. The total of all the formal charges of each atom will always equal the charge on the entire structure (0 for neutral molecules).

Dot structure for H2O .. H – O – H ¨ FC (H) = 1-1-0 = 0 FC (O) = 6 – 2 – 4 = 0 This is excellent, all the FCs are 0!

DON’T EVER STOP AND THINK ABOUT WHERE THE ELECTRONS CAME FROM!!!

Clicker Choose the best Lewis Dot Structure for: SCl2

N2S

Another example Let’s try CO2

CO2 CO2 Total number of valence electrons = 4 from carbon + 2x6 from oxygen = 16 Central Atom? Either C or O could be a central atom. C is more likely (to the left, to the left, to the left…)

CO2 CO2 16 total valence electrons O – C – O Fill out the octets .. .. .. :O – C - O: ¨ ¨ ¨

CO2 16 total valence electrons .. .. .. :O – C - O: ¨ ¨ ¨ Structure has 20 electrons in it. Too many! I need to lose 4 electrons. What’s the best way to do that? Make 2 bonds – each new bond costs 2 electrons

CO2 :O = C = O: ¨ ¨ Structure has 16 electrons in it. Just right! Notice, this works because there are 2 ways to count the electrons: • When I count the total # of electrons, I count each electron once. • When I count the electrons for each atom, I count the bond twice (once for each atom in the bond)

CO2 :O = C = O: ¨ ¨ Is this the only structure I could have drawn? I only needed two new bonds, I didn’t specify where they needed to go! .. :O C - O: ¨ .. :O - C O: ¨ Which is correct?

Choosing between different structures? The first test is formal charge: :O = C = O: ¨ ¨ FC (O) = 6 – 2 – 4 = 0 FC (C) = 4 – 4 – 0 = 0 .. :O C - O: ¨ FC (left O) = 6 – 3 – 2 = 1 FC (C) = 4 – 4 – 0 = 0 FC (right O) = 6 – 1 – 6 = -1 Based on formal charge the upper structure is the better one.

Are these even different? .. :O C - O: ¨ .. :O - C O: ¨ Depends on what I mean by different!

Are they different? .. :O1 C – O2 : ¨ .. :O1 - C O2 : ¨ If I label them, I can see a difference. (Isotopic labeling). If I don’t label them, they are interchangeable, just rotate the top one to get the bottom one.

Resonance .. :O1 C – O2 : ¨ .. :O1 - C O2 : ¨ Structures that are identical, but differ only in the arrangement of bonds are called resonance structures. Resonance is always GOOD!

Resonance When you have resonance, the real structure is not any one of the individual structures but the combination of all of them. You can always recognize resonance – there are double or triple bonds involved. If you take the 3 different CO2 structures, the “average” is the original one we drew with 2 double bonds.

Resonance Resonance is indicated by drawing all resonance structures, separated by “ ” .. .. :O C - O: :O - C O: :O = C = O: ¨ ¨ ¨ ¨ But this is not necessary in this case, as the last structure is also the combination of the 3 structures

Nitrite ion Draw the Lewis Dot structure for NO2- How many valence electrons? N has 5, O has 6, but there’s one extra (it’s an ion!) 5 + 2 (6) = 17 valence electrons + 1 extra = 18 valence electrons

Nitrite LDS What’s the central atom? Nitrogen O – N – O .. .. .. :O – N - O: ¨ ¨ ¨ Total number of electrons? 20 electrons – too many

Nitrite LDS .. .. .. :O – N - O: ¨ ¨ ¨ How do you fix the problem? Make a bond .. .. .. :O = N - O: ¨ What do you think? RESONANCE

Nitrite LDS .. .. .. .. .. .. :O = N - O: :O - N = O: ¨ ¨ What’s the real structure look like? It’s an average of those 2. Kind of 1-1/2 bonds between each N and O! In fact, if you measure the bond angles in nitrite, you find that they are equal (a double bond would be shorter than a single bond)

Let’s try another… CO32-

Lewis Dot Structures